The present invention relates generally to multiplexed data buses, and more particularly, to an access method for a multiplexed data bus which employes a single signal to control the operation of systems attached thereto.
In present ground command, control, communications, and intellingence information systems, the traditional hierarchical architecture under control of a central computer results in an inflexible system configuration. A large percentage of the development, procurement, and maintenance costs of such systems are attributable to interface circuitry, cables, and connectors, and the like. Typical system specifications require a system architecture capable of continued operation during the failure and repair of individual units, and during operational reconfiguration of the system. This necessitates an interconnect structure containing no critical nodes or central control elements. These requirments may be met by utilizing an interconnect system employing a data bus which is shared by all units each of which comprises a standard bus interface.
In a conventional large scale system, the interconnection of computers, processors, displays, and peripheral units requires a unique hierarchical arrangement where the function of a unit is determined by its position in the interconnecting topology. Moreover, the associated interfaces of any one type of unit would change, depending on its location in the topology. The conventional system contains a central computer connected to mini computers through interface units, which in turn communicate with peripherals, displays and communication equipment by way of specialized controllers.
To ensure continuous operation when unit failures occur, the conventional system is redundant, in that a plurality of complete systems are provided to compensate for unit failures. As the number of units in the system increases, the number of point to point interconnections increases at a greater rate and so do the necessary interface units, cards, and cables. The configuration of the interconnecting elements in the topology may vary from system to system, and hence integration of these various systems is most complex.
To circumvent some of the problems associated with this conventional type of system, a single multiplexed data bus may be employed to interconnect each of the units in the system. The central computer may be eliminated by utilizing distributed processing. The computer units are required to interface only with the data bus. Any computer may perform any role in the system since each computer is connected directly to all other computers, peripherals and displays by way of the multiplexed data bus. Continued operation during the failure of a unit only requires an additional backup unit. The result is reduction in the types and numbers of units in the system, and the number of interface cards per unit. As a result, acquisition and life cycle costs are reduced because inventory and maintenance requirements are alleviated.
Multiplexed data buses are currently available, and include those defined in MIL-STD-1553 and IEEE 488 specifications. Although circuitry to employ such data buses may be integrated into a particular unit to provide for standarization, the performance of these data buses in terms of bit rate, transmission distance, and number of units which may be connected to the bus have generally proved inadequate. On the other hand, current high performance bus circuitry is so complex that imbedding the bus interface circuitry into all of the units of a system is impractical. In general, existing bus control architecture designs are unacceptable because control resides in a bus controller or interconnect that represents a critical node.
In any multiplexed bus scheme, the problem arises as to which unit should transfer data along the data bus at any one time. In prior art systems, the determination of the next unit to transmit is accomplished by one of three methods: central control; contention; or token passing. Existing multiplexed bus schemes using central control, such as MIL-STD-1553, solve this problem by designating a particular bus interface as the bus controller and this bus interface controls the transmission by the other bus interfaces on the multiplexed bus. If the bus controller fails, the entire data bus is rendered inoperative.
Using a contention scheme, the need for central control is eliminated by allowing bus interfaces to transmit any time the multiplexed bus is idle. When more than one bus interface is waiting for the bus to become idle, the possibility exists that two or more signals may be transmitted at substantially the same time, and hence colliding with one another on the data bus. In essence each signal jams the other and hence the transmission is garbled and unusable. The occurance of collision slows down system operating speed. In systems which are operating at extremely high data rates, this collision problem is a limit on the system operational speed.
In order to eleviate the problems of contention, systems have been designed which pass a control word, or token, containing a sequence identifier, along the data bus. A token identification is assigned to each unit, allowing each unit to recognize and accept its token. Upon accepting the token, that bus interface must pass the token to the next bus interface after transmitting its data (if any) by transmitting a token with the identifier of the next bus interface in the sequence. In this manner access to the bus for the purpose of transmitting data is sequentially passed from bus interface to bus interface. However, in prior token passing schemes, the disabling or loss of a unit on the bus results in the disruption of the multiplexing and in essence the token gets lost and data can not be transmitted.
Thus, it would be an improvement in the data transmission art to provide a method of transmission control which allows the addition or deletion of units from the system without interruption of data transmission. It would also be an improvement in the art to provide a data bus which does not require the use of a designated unit to control data transmission through the bus.